A weld bead that is formed on a metal workpiece typically includes a number of non-metal inclusions. These non-metal inclusions can negatively interfere with the quality and strength of the weld bead. In some instances, these non-metallic inclusions can increase the incidence of fracturing of the weld bead. As a result, weld bead analysis is commonly conducted on weld bead samples to determine the quality of the formed weld bead.
There are a variety of techniques that have been used to determine the number, type and size of non-metallic inclusions in a metal material. Examples of these techniques include 1) polishing followed by microscopic analysis; 2) ultrasonic analysis; 3) X-ray analysis; 4) magnetic analysis; 5) spark discharge analysis, etc. Several of these techniques are disclosed in patent and Publication Nos. US 2004/0176210; U.S. Pat. No. 6,555,063; U.S. Pat. No. 6,014,024; JP 2004-177168; JP 9-43152; JP 9-33516; and JP 8-184537, all of which are incorporated herein by reference.
The most common technique for analyzing non-metallic inclusions in weld samples involves the extraction of such non-metallic inclusions from the weld samples. The traditional method of non-metallic inclusion extraction involves etching the weld bead with a 5% solution of nitric acid in methanol (nital), lightly coating the etched weld bead with carbon, and then electro-etching the carbon coated weld bead with a 5% solution of methanol and hydrochloric acid. During the electro-etching process, the weld metal beneath the layer of carbon is removed by the hydrochloric acid resulting in the carbon film floating to the surface of the electro-etching solution. The carbon film is caught on a copper grid for later analysis. One of the principal drawbacks of this process is that as the carbon film is caught on the copper grid, the carbon film may not be in the proper orientation on the copper grid to observe the non-metallic inclusions that have adhered to the carbon film surface. As such, when the carbon film is examined and viewed by scanning electron microscopy (SEM), the side of the carbon film that includes the non-metallic inclusions may not be properly oriented, thus imaging of the non-metallic inclusions on the carbon layer can be poor. The thickness of the carbon film can also adversely affect the images of the non-metallic inclusions on the carbon film. Other drawbacks to this method of analyzing non-metallic inclusions in weld metal are that the technique is very time consuming, and requires significant skill in handling the delicate carbon film.
In view of the current state of the art for analyzing non-metallic inclusions in weld metal, there is a need for a method that removes non-metallic inclusions from a metal material that is simpler and less time consuming, and which results in better imaging of the non-metallic inclusions.